php regex with named subpattern: get rid of an extra term in the output

Context

i’m trying to extract coefficients from a random given quadrtic equation string (via an api i don’t have a control on it) following this pattern:

ax² + bx + c = 0  # note blanc spaces are optional

obviously, b and c coefficients can be of zero value but not a, meaning bx and c terms can be optional, otherwise we are out of scope of resolving a quadratic equation.

i came up with the following regex which deals with the given pattern:

/(?P<a>-? ?\d*)x² ?((?P<b>[+-] ?\d*)x ?)?(?P<c>[+-] ?\d+)? ?= ?0/

i used a named subpattern ?P<..> to differenciate between a, b and c coefficients, refer to this topic https://www.php.net/manual/en/function.preg-match.php section Example #4 Using named subpattern

e.g output:

array(
  0 => -2x² + 5x - 1 = 0
  a => -2
  1 => -2
  2 => + 5x  *** the extra term i'm trying to get rid of it. 
  b => + 5
  3 => + 5
  c => - 1
  4 => - 1
)

to test it online: https://www.phpliveregex.com/

Problem

it works fine as expected, but it yields an extra term, the bx term (+ 5x in the given e.g) which i’m confused and i’m trying to understand why and if it’s possible to tweak the regex to eliminate it.

>Solution :

The extra term comes from the (...)? around the named group b. You can make that a non-capturing group with the ?: prefix.

/(?P<a>-? ?\d*)x² ?(?:(?P<b>[+-] ?\d*)x ?)?(?P<c>[+-] ?\d+)? ?= ?0/